1. Field of the Invention
This invention relates to sludge topography measurement in containers of liquids, particularly in oil tanks.
2. State of the Art
Large oil tanks are used to store crude oil, both when the crude oil is in transit and when it is waiting to be fed to a refinery. The oil storage tanks are typically about 20 meters high and 80 to 100 meters in diameter. Crude oil usually contains significant quantities of water: in order to remove this water, the oil is allowed to settle in the oil storage tank, so that the water falls to the bottom of the tank and can then be drained away. However, as well as water, crude oil also contains some organic materials, in the form of relatively dense waxes which also settle on the bottom of the oil tank, forming a non-uniform deposit of sludge which builds up progressively over time to form a layer which, in some places, can be several meters deep.
The layer of sludge is undesirable for two reasons. Firstly, the sludge can trap pockets of water so that the water cannot be drained away. Secondly, the oil tank usually has a deck or roof which floats on the surface of the oil and has several legs on its underside, but these legs can become lodged in the sludge when the tank is empty: it can then be very difficult to release the floating deck and the deck may be stressed and damaged as the tank is refilled.
In view of these problems, it is necessary for the sludge to be removed periodically. This is achieved by emptying the tank of its oil so that its floating deck rests with its legs on the bottom of the tank: the legs are typically 2 meters long, leaving sufficient space under the deck for personnel to enter and remove the sludge. However, before allowing the floating deck to settle in this manner, it is necessary to determine the depth and distribution of the layer of sludge, in case for example there are high spots on which the deck (or its legs) may come to rest, leaving other parts of the deck unsupported and therefore under stress.
At present, the most widely used method of determining the depth and distribution of the sludge is to simply lower a line, in the manner of a dipstick and from the top of the deck, down hollow collars of the floating deck, in which collars the support legs are normally fixed. The person performing this task is able to sense when the line reaches the surface of the sludge: by repeating the exercise at a large number of the support legs, information as to the depth and distribution of the sludge is derived. However, this method is time-consuming and often inaccurate as there are insufficient support legs to allow enough measurements to be taken to cover the whole area reliably.
Another method which has been proposed involves the use of an infra-red detector which is directed onto the outside of the tank and relies upon a difference in temperature between the sludge and the bulk oil above the sludge. However, this method only provides an indication of the depth of the sludge at the tank wall and does not indicate the distribution of the sludge across the whole of the bottom of the tank, as is required.